In drilling, producing or injecting hydrocarbons or other fluids within a subterranean formation, it is often necessary or desirable to determine the in-situ formation stresses. While drilling a borehole, in-situ stress can be used to determine the maximum mud weight (above which lost circulation or blow out risks are unacceptable), and the minimum mud weight to avoid the risk of borehole collapse. While completing a well, in-situ stress can form the basis for determining whether the borehole needs to be gravel packed and for engineering a hydraulic fracture treatment.
Two basic techniques for determining in-situ stress are known: 1) direct measurements of force and area; and 2) measurement of displacements induced by stress. Displacement measurements may be accomplished in conjunction with cuts to relieve in-situ stress. One cutting technique creates a slot in the borehole face formation, completely relieving the slot surfaces of the stresses across them. The relief results in expansion and/or displacement which can be measured. The slot surfaces can also be forced back into the undisturbed condition and the force (and stress if area is known) and strain can be measured. However special cutting tools are required, and creep, grouting and cancellation effects can introduce serious errors to the stress and strain determinations.
Force techniques apply stress to the surface of a borehole, producing deformations (strains) an fractures which can be used to determine the direction of the minor principal stress. A fluid pressure fracturing (hydrofracturing) technique obtains pressure data which can be used to determine some of the in-situ stress. Fluid flow and pressure is increased to a sealed off axial section of the borehole until fractures develop and propagate. The flow is reduced and the pressure is measured, called the instantaneous shut-in pressure (ISIP). Since the borehole at depth is typically under compressive load and the tensile strength capability of rock is generally small, the increasing hydraulic pressure relieves the compressive in-situ stress until tensile fracture occurs. Fracture is typically in a direction perpendicular to the least principal horizontal stress.
It is known that the ISIP after initial fracturing is related to the least principal stress. This phenomenon (shut in pressure at after initiation/opening of fracture determining the least principal stress) has been well-demonstrated by laboratory and theoretical studies. The methods of calculation are described in two references: 1) Zoback, M. D. and Haimson, B. C., "Chapter 15, Status of the Hydraulic Fracturing Method for In-Situ Stress Measurements" in: Goodman, R. E. and Heuze, F. E., Editors, "Issues in Rock Mechanics, Proceedings Twenty-Third Symposium on Rock Mechanics, The University of California, Berkeley, Calif., Aug. 25-27, 1982" (New York, Society of Mining Engineers of the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., 1982), pp 143-156; and 2) a paper by Hickman, S. H., and Zoback, M. D., entitled "The Interpretation of Hydraulic Fracturing Pressure-Time Data for In-Situ Stress Determination" in: U.S. Geological Survey, "Hydraulic Fracturing Stress Measurements" (Washington, National Academy Press, 1983), pg 44-54, said chapter and paper being incorporated hereinto by reference.
The instantaneous shut-in pressure (direct injection of fluid is halted and pressure measured) rather than breakdown pressure (fluid pressure measured when fracture occurs) has been used in these incorporated references to determine in-situ stresses because of several factors. Theoretical factors avoided by using ISIP include accounting for injection pressure loss or frictional flow of fluids into the formation or fractures, potential fracture propagation away from the borehole, and the presence of pre-existing fractures at the borehole. Since the hydraulic fluid flow cannot be shut in instantaneously and the presence of other factors is generally unknown, the ISIP measurements after fracture initiation may no longer be suitable for determining the least principal stress near the borehole. In addition to these theoretical and practical problems is determining in-situ stresses, shut-in pressure may be indistinct (e.g., pressure may rapidly decrease to zero when injecting fluid flow is stopped).
These references and others have attempted to correct for these factors/problems in measuring ISIP and determining in-situ stress. Correction methods include multiple injection flows and repeated measurements of shut-in pressures, pressure decrease rate analysis, pressure increase/build up upon re-pressurization analysis, and fracture re-opening pressurization analysis. In addition, a separate determination of fracture magnitude and direction may be required.
All of the current in-situ stress measurements methods require special downhole tools or fluid injection and procedures. If the in-situ stress determinations are required during a drilling operation (including completion, logging and related activities), the drilling operations must be interrupted, operational equipment removed, special equipment installed, pressure, force, and/or displacement data obtained, in-situ stress calculated, the special equipment removed, and operational equipment replaced.
None of the current or alternative approaches known to the inventor eliminates the special equipment installation, removal, and interruption problems in order to obtain in-situ stress measurements. Drilling interruption may also cause additional problems, such as thermal soak or shock, cave in, settling, and stuck pipe.